Mammoth Uncensored Word List:
33 - 15 letter words starting with so

socialistically sociobiological sociobiologists sociocentricity socioculturally sociohistorical sociolinguistic socioscientific softheartedness solderabilities soldierlinesses solemnification solidifications solipsistically solitudinarians solubilisations solubilizations solvatochromism somatologically somnambulations sonoluminescent soothfastnesses sophisticatedly sophistications sorrowfulnesses soundlessnesses southeasterlies southeasterners southeastwardly southerlinesses southwesterlies southwesterners southwestwardly